The present invention relates to a method of manufacturing a chemical compound semiconductor monocrystal grown by LEC (liquid encapsulated Czochralski) method, and more particularly to a method of manufacturing semi-insulation GaAs monocrystal used for electronic devices such as Hall Sensors, high-frequency EFTs, etc.
In the case where SI (semi-insulation) GaAs monocrystal is manufactured in accordance with LEC method, even if impurities are not positively introduced for reason of monocrystal formation, acceptor impurities such as carbon, boron, etc. inevitably exist in the monocrystal. In the manufacturing process of electronic devices using SI GaAs substrate, it is required to reduce these impurities as much as possible, because these impurities deteriorate the device characteristics.
In general, when the impurities are carbon or boron, it is so far well known that there exists mutual relationship between the moisture concentration of a liquid encapsulating compound (referred to as sealing compound, hereinafter) B.sub.2 O.sub.3 (used to pull up the monocrystal) and the impurity concentration in the crystal. Therefore, it has been so far adopted a method of preventing carbon or boron from being mixed with the crystal by controlling the moisture concentration in the sealing compound B.sub.2 O.sub.3.
In practice, there exists such a tendency that the higher the moisture concentration in the sealing compound B.sub.2 O.sub.3 is, the smaller will be the concentration of the impurities (carbon or boron) in the crystal. Further, irrespective of the moisture concentration in the sealing compound B.sub.2 O.sub.3, it has been known that the carbon concentration is high near the crystal head but low near the crystal tail and the boron concentration is low near the crystal head but high near the crystal tail.
Therefore, in the case of the SI GaAs wafer used for the Hall Sensors, FETs, HEMTs, etc., it is particularly necessary to suppress the mixture of impurities of carbon and boron with the crystal, because these impurities cause characteristic dispersion and thereby the characteristics deteriorate.
Conventionally, the impurities such as carbon or boron have been suppressed by controlling the moisture concentration in the sealing compound B.sub.2 O.sub.3.
In the conventional method of suppressing the impurities such as carbon and boron, however, there exists a problem in that when the carbon concentration is decreased at the crystal head, the carbon concentration decreases excessively at the crystal tail so that the semi-insulation characteristics are not obtained. In other words, there exists a limit of reducing the carbon concentration, as a matter of course.